Typically a mobile device in mobile communications network needs to detect a number of surrounding cells. The reference signal receive power (RSRP) and reference signal receive quality (RSRQ) of each detected cell are two key measurement metrics used during the initial cell selection procedure. After camping on a cell, the mobile device reports the RSRP and RSRQ repeatedly to the network to facilitate the mobility management, e.g., network controlled handover. Moreover, the RSRP of a neighboring cell reflects the perceived interference level from the respective neighboring cell, as such, it may be useful for the mobile device to determine whether to adopt further advanced signal processing algorithms or as to cancel/mitigate the interference and improve the link reliability.
It is obvious that the measurement accuracy of RSRP/RSRQ depends on the amount of reference signals present in the time-frequency plane, for example in an OFDM system such as LTE the amount of cell specific reference signals (CRS) employed during the measurement, i.e. measurement bandwidth and number of sub-frames. To achieve an optimized design in terms of performance and power/are a consumption trade-off, the mobile device aims to estimate the RSRP with sufficient accuracy while using as less time-frequency resources as possible, for example in an LTE system as less physical resource blocks (PRB) and sub-frames as possible. Furthermore, it is desired to employ only 6 central PRBs within the system band width in a sub-frame to perform the RSRP measurement and fulfill the relevant performance accuracy.
To facilitate further explanations, the following notation shall be used in this disclosure.
K: Number of CRS resources in the employed measurement time-frequency resources grid.
hk: channel frequency response at the k-th CRS resource, where k=0, . . . , K−1
ĥk: channel frequency response estimate at the k-th CRS resource, where k=0, . . . , K−1
zk: channel frequency response estimate error, and assumed to be independent and identically distributed random variable with complex Gaussian distribution of with zero mean and variance σN2.
It follows thatĥk=hk+zk  (1)
Defining vectors ĥ=(ĥ0, ĥ1, . . . , ĥk−1)T, h=(h0, h1, . . . , hk−1)T and z=(z0, z1, . . . , zk−1)T, equation (1) can be simplified asĥ=h+z  (2)
In a current solution, the mobile device firstly calculates the power of channel estimate from each time-frequency resource of the reference signal within the measurement time-frequency resources grid, and then averages over all these power estimates to obtain the RSRP. Hence, typically RSRP estimation is performed by auto-correlating channel coefficients of an estimated channel. This method can be expressed as follows.RSRP=∥{circumflex over (h)}∥2/K  (3)
As the estimate involves multiplying a channel coefficient by its complex conjugate, it can be observed that an estimate of the reference signal received power at a specific time-frequency resource is severely corrupted when significant noise is present at this time-frequency resource.
Hence, what is needed is a solution leading to a more accurate estimate of the reference signal received power.